1. Field of the Invention
This invention generally relates to object detector systems and, more particularly, to an object detector of the type including a transmitter coil and a pickup coil between which an object to be detected passes.
2. Background of the Invention
In object detectors of the type having a transmitter coil and a pickup coil, the transmitter coil is connected to a source of alternating current and the pickup coil is connected to a detector which detects any change in voltage as a result of a metallic or magnetic object passing between the two coils. The pickup coil voltage depends on the operating frequency, the transmitter coil current and the relative geometry between the transmitter coil and the pickup coil. These values are, in turn, a function of the transmitting coil inductance, the applied voltage, and the tuning capacitor capacitance.
This type of object detector has worked well and reliably in a variety of environments including portable metal detectors and manufacturing and inventory control. However, it has been observed that if the operating frequency and/or the transmitter coil current change, the pickup coil output changes, sometimes giving a false indication that an object is present.
Object detectors of this type are disclosed, for example, in several Turner U.S. Pat. No. 4,300,097 discloses a metal detector with ferrous and non-ferrous metal identification which includes circuitry to null out the effects of mineralized soil. Dykstra et al. U.S. Pat. No. 4,325,027 discloses a metal detector which includes circuitry for tracking the amplitude of any component of the received signal attributable to distributed mineral materials so that such a component can be cancelled. Randolph, Jr., U.S. Pat. No. 4,344,034 discloses a metal detector including two detecting circuits, one providing a selective response to metal objects and the other providing a response to metal objects while excluding ground effects from the output of the first detecting circuit. Other U.S. patents of interest include Gifford No. 4,486,713 and Kerr No. 4,563,645.
While Turner, Dykstra et al. and Randolph, Jr. are examples of object detectors which include some sort of compensation to avoid false indications, the prior art does not recognize the problem that a change inoperating frequency and/or transmitter coil current can cause in this type of detector.